Buprenorphine is a partial mu agonist under consideration by the FDA for use in opioid substitution therapy. It will join methadone and I-alpha-acetylmethadol (LAAM) as medications available at treatment centers. Buprenorphine is also being considered for private physician dispersal; which will greatly expand access to substitution therapy. All three of these drugs are cytochrome P450 (P450) 3A4 substrates. Drug interactions are a major cause of serious adverse drug effects and P450 3A4 is a major site of drug interactions. Many known drug interactions with methadone are due to its metabolism by P450 3A4. The proposal funded during the last period (R01 DA 10100-02) tested the hypothesis that the 3A4 inhibitor, ketoconazole, would inhibit the metabolism of LAAM to its more active metabolites, norLAAM and dinorLAAM, and thereby diminish its therapeutic effect. Data from that study demonstrate both in vitro and in vivo evidence of significant drug interactions providing support for this hypothesis. Since buprenorphine is nearing market launch, and few studies exist on its potential for serious drug interactions, we now hypothesize that determination of enzymes involved in the in vitro hepatic metabolism of buprenorphine will allow prediction of in vivo drug interactions of clinical significance. While testing this hypothesis, the fact that buprenorphine and its metabolite, norbuprenorphine, are extensively glucuronidated, and preliminary data that suggest other metabolites are produced by P450s must be taken into consideration. Therefore, to test this hypothesis we will: 1) extend our current liquid chromatographic-tandem mass spectrometric (LC-MSMS) method for buprenorphine and norbuprenorphine to include their glururonides; 2) use LC-MSMS to identify metabolites of buprenorphine generated in human liver microsomes (HLM) and cDNA-expressed P450s; 3) determine the quantitative involvement of specific P450s in these pathways using kinetic analyses, specific inhibitors, specific antibodies and correlational analysis in HILM and cDNA-expressed P450s; determine the qualitative involvement of specific UDP-glucuronosyl transferases (UGTs) using HILM and cDNA-expressed UGTs; and 4) determine the in vitro induction of buprenorphine phase I and phase II metabolism in primary cultures of human hepatocytes. We anticipate these studies will provide the evidence to support clinical studies to more thoroughly test our hypothesis.